A lead frame for a semiconductor device is used to mount a semiconductor device on a substrate or the like. A semiconductor element is mounted on the lead frame and encapsulated in a resin material together with the lead frame. Japanese Patent No. 2648353 and Japanese Examined Patent Publication No. 1-35503 each describe a conventional lead frame. For example, a lead frame includes a die pad on which a semiconductor element is mounted, an inner lead electrically connected to the semiconductor element, and an outer lead formed integrally with the inner lead and used as an external connection terminal when mounting a semiconductor device on the substrate or the like. In this type of a lead frame, a plating layer is formed on the surfaces of the die pad and the inner lead to improve the mounting characteristics of semiconductor elements and the connectivity to metal wires (bonding wires). This type of a lead frame is formed through, for example, the manufacturing process described below.
Referring to FIG. 13A, a mask 110 is first formed on a flat metal plate 100. The mask 110 covers, for example, a portion of the metal plate 100 that is used as an outer lead. Then, in the step illustrated in FIG. 13B, electrolytic plating is performed using the mask 110 as a plating mask and the metal plate 100 as a power supplying layer to form a plating layer 101 on the upper surface of the metal plate 100 located outside the mask 110. In the step illustrated in FIG. 13C, stamping is performed to form an opening 100X in the metal plate 100. The opening 100X defines a die pad 102, inner leads 103, and outer leads 104. The plating layer 101 is formed on the upper surface of the die pad 102 and the upper surface of a distal portion 103A of each inner lead 103.
In the manufacturing method described above, after the plating layer 101 is formed, a coining process is performed on the distal portions 103A of the inner leads 103. The coining process may change the crystal structure at the surface of the plating layer 101 (e.g., silver plating layer). For example, when the crystal structure changes and increases the hardness of the plating layer 101, connections of metal wires may become defective and the bonding strength of metal wires may decrease. The coining process is a process that presses and flattens the surface of a processed subject with a stamping die.